In the isolated perfused retina of the toad, intracellular work in rod outer segments will be used to determine whether calcium can mimic the action of light sufficiently well to support the hypothesis that it is an internal transmitter involved in generation of the rod receptor potential. The functional relation between outer and inner segments will be studied by intracellular recordings in these two cellular sites. The site and nature of synaptic actions involved in summative interactions between rods of the snapping turtle will be studied by simultaneous recordings in two connected rods, and by voltage clamping with double barrelled electrodes. A noise analysis of the receptor potential will be undertaken, in part to determine the role of summative rod interaction in reducing noise in the receptor signal.